Analog single sideband modulators are undesirable because they tend to be structurally complex, require manual alignment procedures and suffer from local oscillator (LO) leakage and distortion. In addition, they are susceptible to drift over time and temperature, and they tend to have inflexible designs that cannot readily support variety or complexity in modulating functions.
Digital modulators solve many of the problems associated with analog modulators. Generally, digital modulators use numerically controlled oscillators (NCOs) that are digitally controlled to generate a modulated signal. The modulated signal may then be up-converted as needed for transmission. Digital modulators offer great flexibility, minimize alignment procedures, are generally much less complicated and provide generally better LO leakage performance than analog modulators.
Digital modulators pose their own set of problems. One particularly vexing problem is that sampling images tend to appear in the modulated signal. Sampling images are spurious frequency components that are offset in frequency from the modulation center frequency F.sub.c by approximately the clock rate of the NCO. When the center frequency to bandwidth ratio of the modulated signal is relatively low, these sampling images are difficult to remove with realizable filters without likewise attenuating the modulated signal itself. Thus, digital modulators tend to suffer from spurious outputs.
The conventional solution to the sampling image problem is to clock the NCO at a very high frequency. This further offsets the sampling images from the center frequency F.sub.c. The further the sampling images are offset from the center frequency F.sub.c, the more easily they can be filtered out using realizable filters.
The conventional solution imposes a severe and undesirable impact in applications where radiation susceptibility and/or power consumption are concerns, such as in spacecraft-mounted transmitters. Often, an NCO that operates at a frequency that is high enough to permit sampling images to be removed by realizable filters must be implemented using a high speed semiconductor component, such as a GaAs semiconductor, that has poor radiation resistance and consumes a large amount of power.